The present invention relates to the growth of silicon carbide for semiconductor purposes, and to the seeded sublimation growth of large, high quality silicon carbide single crystals. The invention particularly relates to improvements that reduce the defect density and polytype changes in large single crystals grown using seeded sublimation techniques.
Silicon carbide has found use as a semiconductor material for various electronic devices and purposes in recent years. Silicon carbide is especially useful due to its physical strength and high resistance to chemical attack. Silicon carbide also has excellent electronic properties, including radiation hardness, high breakdown field, a relatively wide band gap, high saturated electron drift velocity, high temperature operation, and absorption and emission of high energy photons in the blue, violet, and ultraviolet regions of the spectrum.
Single crystal SiC is often produced by a seeded sublimation growth process. In a typical silicon carbide growth technique, a seed crystal and a source powder are both placed in a reaction crucible which is heated to the sublimation temperature of the source and in a manner that produces a thermal gradient between the source and the cooler seed crystal. The thermal gradient encourages vapor phase movement of the materials from the source to the seed followed by condensation upon the seed and the resulting bulk crystal growth. The method is also referred to as physical vapor transport (PVT).
In a typical silicon carbide growth technique, the crucible is made of graphite and is heated by induction or resistance, with the relevant coils and insulation being placed to establish and control the desired thermal gradients. The source powder is silicon carbide, as is the seed. The crucible is oriented vertically, with the source powder in the lower portions and the seed positioned at the top, typically on a seed holder; see U.S. Pat. No. 4,866,005 (reissued as No. RE34,861). These sources are exemplary, rather than limiting, descriptions of modern seeded sublimation growth techniques.
It has proven difficult, however, to produce large, high quality bulk single crystals of silicon carbide by the typical seeded sublimation techniques. Large crystals grown according to the typical methods suffer from the presence of a large number of defects. The 150 available polytypes of SiC raise a particular difficulty. Many of these polytypes are very similar, often separated only by small thermodynamic differences. Maintaining the desired polytype identity throughout the crystal is only one difficulty in growing SiC crystals of large sizes in a seeded sublimation system.
Preferred polytype SiC crystals for semiconductor applications are 4H and 6H. These crystals are preferably grown with a convex surface to better enable the maintenance of polytype registry. The convex surface consists of a series of steps, from the peak of the dome to the sides of the crystal. Preferably, the steps are microsteps—those with a depth of less than 1 μm, more preferably less than about 500 nm. Microsteps promote consistent polytype registry during crystal growth, because polytype information from the seed is readily available to the depositing vapors. When the convex surface includes macrosteps—those with a depth of 1 μm or greater, this implies that there are larger terraces on the crystal surface that do not contain microsteps. This, in turn, leads to a loss of polytype registry.
Failure to maintain the polytype registry of a crystal during growth will typically result in a crystal having a high level of defects. Defects that can result from polytype changes include micropipes and dislocations. High concentrations of micropipe defects cause significant problems in limiting the performance characteristics of devices made on substrates fabricated from the crystals. For example, a typical micropipe defect density in some commercially available silicon carbide wafers can be on the order of 100 per square centimeter (cm−2). A megawatt device formed in silicon carbide, however, requires a micropipe free area on the order of 0.4 cm−2. Thus, obtaining large single crystals that can be used to fabricate large surface area devices for high-voltage, high current applications remains difficult.
It would therefore be desirable to develop a method for reducing the presence of macrosteps on the growing surface of a bulk single crystal of SiC in order to produce micropipe-free, high quality bulk single crystals.